1. Field of the Invention
This invention relates to a perpendicular magnetic recording medium mounted in various magnetic recording devices. More specifically, this invention relates to a perpendicular magnetic recording medium for hard disk drives (HDDs) used as external storage devices in computers, audio and video equipment and the like.
2. Description of the Related Art
Since 1997, the recording density of HDDs has risen rapidly at an annual rate of from 60 to 100%. As a result of this remarkable growth, the in-plane magnetic recording methods which have heretofore been used are approaching the limits of density increases. In light of this state of affairs, in recent years perpendicular magnetic recording methods enabling higher densities have attracted much attention, and have been the object of vigorous research and development. And since 2005, there has been the beginning of commercialization of HDDs adopting perpendicular recording methods among a portion of device models.
Perpendicular magnetic recording media include, at least, a magnetic recording layer of a hard magnetic material, and optionally a under layer to orient the magnetic recording layer in the target direction; a protective film to protect the surface of the magnetic recording layer; and a under layer of soft magnetic material, serving to concentrate in the magnetic recording layer the magnetic flux generated by the magnetic head used in recording in the magnetic recording layer. In order to improve the signal output-noise ratio (S/N) of the magnetic recording medium, the basic characteristics of the magnetic recording medium must be improved. To further raise recording densities, improved thermal stability of signals recorded in perpendicular magnetic recording media is sought. And in order to raise thermal stability, increases in the magnetic anisotropy Ku of the magnetic recording layer are sought. In order to increase the magnetic anisotropy, studies are being conducted using crystal magnetic anisotropy as represented by L10 and other structures, or using interface magnetic anisotropy employing multilayer films.
A technique has been proposed in which, by inserting a second intermediate layer to relax the misfit in lattice constants of a first intermediate layer formed of Ru or another nonmagnetic metal and the magnetic recording layer having a granular structure, the easy axis of magnetization of magnetic crystal grains in the granular structure are aligned (see for example Japanese Patent Application Laid-open No. 2002-208126). In this proposal, it is held to be desirable that the misfit between the lattice constants (a axis and c axis) of the second intermediate layer and the magnetic crystal grains of the magnetic recording layer, and the misfit between the lattice constants (a axis and c axis) of the first intermediate layer and second intermediate layer, be held to 3% or less. As material for the second intermediate layer, it is stated that alloys obtained by adding Nb, Ru, W, Pt, or similar to CoCr are used. However, in order to improve the performance of the magnetic recording medium, further improvements have been necessary when reducing the film thickness of an intermediate layer.
Further, a technique has been proposed in a perpendicular magnetic recording medium using a magnetic recording layer having an antiferromagnetic exchange coupled structure in which a nonmagnetic second intermediate layer having a granular structure is provided below a first magnetic recording layer including Ru, and by forming nonmagnetic grains from CoCr alloy not including Ru, and forming grain boundaries from an oxide of a metal (Si, Cr, Ti, W, or similar), Ru included in the second intermediate layer and lower constituent layers is prevented from diffusing into the magnetic recording layer and disturbing the antiferromagnetic exchange coupled structure of the magnetic recording layer (see for example Japanese Patent Application Laid-open No. 2010-27110).